An internal combustion engine capable of using fuels having different properties is mounted in the so-called FFVs (flexible-fuel vehicles). Ethanol-blended gasoline may be typically used for such an FFV internal combustion engine. When ethanol-blended gasoline is used as a fuel for an internal combustion engine, it is necessary to adjust air-fuel ratio in accordance with the concentration of the ethanol in the fuel because ethanol greatly differs from gasoline in calorific value per unit volume. Therefore, internal combustion engines using ethanol-blended gasoline include an ethanol concentration sensor as a fuel property sensor in order to determine the properties of an employed fuel, more specifically, the ethanol concentration. Examples of sensors suitable for the ethanol concentration sensor are a capacitance sensor, an optical transmission sensor, and an optical refractive-index sensor.
The fuel's ethanol concentration measured by the ethanol concentration sensor is used as a parameter for the air-fuel ratio control of the internal combustion engine. This makes it possible not only to obtain a desired torque but also to ensure satisfactory emissions performance without regard to the ethanol concentration in the employed fuel.
As described above, the fuel property sensor in an FFV internal combustion engine plays an important role to ensure the expected performance of the internal combustion engine. However, there is no guarantee that the fuel property sensor functions normally at all times, as is the case with other sensors. Wiring disconnection, short-circuiting, sensor element deterioration, or other abnormality may occur in the fuel property sensor. If, in such an instance, the internal combustion engine is controlled by using output values of the fuel property sensor, the internal combustion engine would fail to operate appropriately in accordance with the properties of the employed fuel, resulting in the performance characteristics of the internal combustion engine such as emissions performance and fuel efficiency to be degraded.
It is therefore desirable that abnormality in the fuel property sensor is accurately detected so as to immediately take an appropriate remedial action such as repair or replacement. In view of the above circumstances, a technology disclosed in JP-A-2010-038052 (hereinafter referred to as Patent Document 1) presets an upper-limit threshold value and a lower-limit threshold value for the output value of an ethanol concentration sensor. When the output value is outside the range between the upper- and lower-limit threshold values, this technology concludes that the ethanol concentration sensor is malfunctioning. In addition, since the output value of the ethanol concentration sensor varies with fuel temperature even when the ethanol concentration is constant, this technology can change the upper- and lower-limit threshold values in accordance with the fuel temperature measured by a fuel temperature sensor.
However, the technology disclosed in Patent Document 1 cannot accurately detect abnormality in the ethanol concentration sensor in all cases. A phenomenon called “stuck” is an abnormality that is likely to occur particularly in the ethanol concentration sensor which greatly affects the control of the internal combustion engine. It is a phenomenon such that the output value of the ethanol concentration sensor becomes stuck at a fixed value. This phenomenon may occur even when the output value of the ethanol concentration sensor is between the upper- and lower-limit threshold values. Therefore, the technology disclosed in Patent Document 1 may fail to detect this phenomenon as an abnormality.
A method of detecting a capacitance temperature sensor being stuck is well-known as described in JP-A-2000-303898 (hereinafter referred to as Patent Document 2). The method described in Patent Document 2 calculates the difference between a maximum water temperature and a minimum water temperature, which are measured by the temperature sensor after startup of the internal combustion engine. If the calculated difference is small, this method concludes that the sensor is stuck. However, it is difficult to apply this method to the detection of an ethanol concentration sensor being stuck. The reason is that, unlike fuel temperature, ethanol concentration in fuel cannot be changed unless operation such as refueling is performed.
When the output characteristics of the ethanol concentration sensor relative to the fuel temperature are taken into account, as in the technology of Patent Document 1, whether the ethanol concentration sensor is stuck can be determined by checking whether the output value of the ethanol concentration sensor varies with the fuel temperature. However, if a fuel whose ethanol concentration is 0% is employed, the output value of the ethanol concentration sensor remains substantially unchanged even when the fuel temperature varies. Therefore, this method cannot determine whether the ethanol concentration in the employed fuel is 0% or the sensor is stuck.
Another method of detecting abnormality in the fuel property sensor is described in JP-A-2008-014741 (hereinafter referred to as Patent Document 3). The abnormality detection method described in Patent Document 3 presumes that the inlet of a fuel tank is provided with a measurement chamber including a fuel property sensor. It is also presumed that the fuel property sensor outputs signals of different levels depending on the presence/absence of fuel at a measurement space within the measurement chamber. When the employed configuration is as described above, no fuel stays in the measurement space during normal operation. Fuel temporarily stays in the measurement space when the fuel tank is being refueled. The signal level of the fuel property sensor then changes reflecting the presence of the fuel in the measurement space. Therefore, if the fuel property sensor does not output an appropriate signal during refueling, it can be concluded that the fuel property sensor is malfunctioning.
However, the technology described in Patent Document 3 has a problem in terms of accuracy in determining the properties of the employed fuel. The fuel properties required as information for controlling the internal combustion engine is the properties of the fuel supplied from the fuel tank to the internal combustion engine, or more specifically, the properties of the fuel injected from an injector. The fuel property sensor in the configuration set forth in Patent Document 3, however, performs determination based on the properties of the fuel supplied to the fuel tank and not those of the fuel injected from the injector. In FFV internal combustion engines, which can use fuels having different properties, the properties of the fuel in the fuel tank do not always match with those of a newly supplied fuel. Therefore, it is highly probable that the fuel properties determined by the fuel property sensor differ from the fuel properties of the fuel injected from an injector in the technology of Patent Document 3. It is therefore difficult to achieve appropriate air-fuel ratio control in accordance with the properties of an employed fuel.
Further, the technology described in Patent Document 3 cannot detect abnormality in the fuel property sensor with adequate accuracy, or more particularly, whether the fuel property sensor is stuck. If, for instance, the output value of the fuel property sensor is stuck at an output level indicative of absence of fuel in the measurement space, it is possible to detect the “stuck” of the fuel property sensor from the output level of the fuel property sensor during refueling. However, if the output value of the fuel property sensor is stuck at an output level indicative of presence of fuel in the measurement space, the output level would not change during refueling, and thus the fuel property sensor will be judged to be operating normally. In other words, the technology described in Patent Document 3 cannot detect a stuck sensor in such case.
As described above, the previously proposed abnormality detection technologies for fuel property sensors cannot detect abnormality in a fuel property sensor with adequate accuracy, or more particularly, cannot detect a stuck fuel property sensor with adequate accuracy.